1. Field of the Art
The present invention relates to an ultrasonic motor wherein the driving force is given by elastic travelling wave excited by a piezoelectric element.
2. Description of the Prior Art
An ultrasonic motor is made with a vibrating stator which comprises a piezoelectric element and an elastic element and a rotor which is disposed to touch the vibrating stator with pressure. For instance, in U.S. Pat. No. 4,562,374, an embodiment using a vibrating stator which comprises a ring-shaped piezo-electric element and an elastic element is described. In the above-mentioned piezoelectric element, two electrode-groups, having a phase difference of 90.degree. in position in a circumferential direction, are provided. By applying two alternating voltages having the phase differences of 90.degree. to these electrode-groups, an elastic travelling wave is excited in the vibrating stator, and thereby the rotor rotates by friction forces.
Travelling wave is represented as follows: ##EQU1## where: .xi.; Amplitude of travelling wave,
.xi.o; Instantaneous value of amplitude, PA1 .omega.; Angular frequency, t; Time, PA1 k; Wave number, x; Position.
Hereupon, when the above-mentioned ring-shaped vibrating stator is used, an area of the piezoelectric element responding to one electrode-group is small, because width in a radial direction is narrow and two electrode-groups are disposed to divide regions in the circumferential direction. Therefore, the driving force for exciting vibrations of one phase is not sufficient, and it is thereby difficult to obtain high driving efficiency.
On the other hand, there is another embodiment of vibrating stator for an ultrasonic motor, as is disclosed by a Japanese Unexamined Published Application Sho No. 60-183982 in which an elastic element and two slices of piezoelectric element are bonded coaxially into three layers. These two slices of piezoelectric element are disk-shaped, each of them is applied with voltage individually, and are superimposed in a manner that phases of excited vibration differ by 90.degree. thereby. Therefore, area of the piezoelectric element which supplies a driving force for excitation is large, and its efficiency is high.
However, since impedances seen from electric terminals of the two slices of the piezoelectric elements are different, when they are driven by voltages of same amplitude and different phases by 90.degree., not only the travelling wave but also standing waves are excited in the vibrating stator as is obvious from equation (b 1). Generation of these standing waves causes lowering of the driving efficiency of the ultrasonic motor.
Futhermore, in another embodiment of vibrating stator, for the above-mentioned Japanese Unexamined Published Application Sho No. 60-183982, what comprises the elastic element and a slice of piezoelectric element, and forms concentric circle-shaped electrodes comprising two regions on the one slice of piezoelectric element, are disclosed. Different phase vibrations are excited by the voltages which are applied to those electrodes. Since this vibrating stator also does not make matching of impedances of two electrode-groups having phase difference of 90.degree. and signals of electric charge excited by the vibration are not taken into account, when the vibrating stator is driven by same amplitudes and different phases by 90.degree., not only the travelling wave but also the standing waves are excited in the vibrating stator.